Magnetoresistive devices or magnetoresistive sensor elements are employed in numerous applications as field sensors and movement detectors (e.g., global positioning systems and navigation systems, magnetometry, compassing, angular and linear position sensors, dead reckoning technologies, vehicle or object detection, telematics or movement direction, rotation direction, rotation speed, velocity of a given object with reference to the sensor arrangement) as well as read sensors in magnetic data storage systems to detect magnetically encoded information stored on a magnetic data storage medium or recording heads to record magnetically encoded information on said storage medium.
In particular, tunneling magnetoresistive (TMR) sensors are employed for the above mentioned applications.
A typical tunneling magnetoresistive (TMR) sensor includes a magnetic tunnel junction (MTJ) including two ferromagnetic layers separated by a tunnel barrier layer. The ferromagnetic layers are commonly referred to as a “fixed” or “reference” layer, in which the direction of magnetization is fixed, and a “free” layer or “sense” layer, in which the direction of magnetization may be switched.
The resistance of an MTJ varies based on the relative directions of magnetization of these layers. For example, when the directions of magnetization of the fixed and free layers are parallel, the resistance may be relatively small, and may become greater when the directions of magnetization are anti-parallel.
For tunneling magnetoresistive (TMR) sensors configured to operate in a current-perpendicular-to-plain mode (CPP) a sense current flowing perpendicular to the plane of the layers of the magnetic tunnel junction (MTJ) experiences a change in resistance which is proportional to the magnetic orientation of a free layer relative to the reference layer.
The resistance change ΔR=RAP−RP, that is the difference between the anti-parallel (RAP) and parallel (RP or R) resistance values, divided by the parallel resistance RP is known as the magnetoresistance (MR) ratio of the magnetic tunnel junction (MTJ) and is defined as (RAP−RP)/RP=ΔR/RP=ΔR/R.
For tunneling magnetoresistive (TMR) sensor applications it is important to have high signal-to-noise ratio (SNR), the magnitude of the SNR being directly proportional to the magnetoresistance ratio (MR ratio=ΔR/R) of the magnetic tunnel junction (MTJ). The signal-to-noise ratio is given by iB ΔR, iB being the bias current passing through the MTJ device. However, the noise obtained by the MTJ device is determined, in large part, by the resistance R of the device. Thus, the maximum SNR for constant power used to sense the device can be obtained if the magnetoresistance (MR) ratio is large.
The resistance R of an MTJ device is largely determined by the resistance of the insulating tunnel barrier layer. Moreover, since the currents pass perpendicularly through the ferromagnetic layers and the tunnel layer, the resistance R of an MTJ device increases inversely with the area A of the device, therefore it is convenient to characterize the resistance of the MTJ device by the product of the resistance R times the area A (RA).
In order to allow the resolution of the continuously increasing bit density of the storage medium, magnetoresistive sensors will need to be shrunk in size, requiring low RA values so that the resistance R of the cell is not too high and sufficiently high sense current densities can be used at acceptable values of reliability of the MTJ device.
Conventionally, the materials used for the insulating tunnel barrier layers are (Magnesium Oxide) MgO or Aluminium Oxide (Al2O3). For MgO or Al2O3 insulating tunnel barriers it has been found that RA increases exponentially with the thickness of the layer. The thickness of the MgO or Al2O3 insulating tunnel barrier layers can be varied over a sufficient range to vary RA by more than eight orders of magnitude, i.e. from more than 2×109Ω(μm)2 to as little as 1Ω(μm)2. For typical MgO based insulating tunnel barriers a RA product of 1Ω(μm)2 to 10Ω(μm)2 is required to withstand current densities in the order of 0.1 MA/(cm)2 to 10 MA/(cm)2. However, for these low RA values, the magnetoresistance (MR) ratio, and therefore the SNR, is typically reduced, in part because of microscopic pin holes or other defects in the ultra thin tunnel barrier layers needed to obtain these very low RA values. Moreover, the ultra thin tunnel barrier layers needed to obtain these very low RA values reduces the barrier reliability.
A widely observed effect in tunneling magnetoresistive (TMR) sensor is the so-called “Neel coupling”, which is a coupling between the sense layer and the reference layer due to magnetostatic interactions between the free poles at the two ferromagnetic interfaces. This coupling is associated with the roughness of the conventional thin insulating barrier and causes a shift in the preferred parallel alignment of the two ferromagnetic layers and thus a deviation from the theoretically expected behavior of the tunneling magnetoresistive (TMR) sensor.
Therefore there is a need in the art for tunneling magnetoresistive (TMR) sensors characterized by a large and stable tunneling magnetoresistance (MR) ratio, a reliable tunnel barrier layer, a low RA value and a reduced Neel effect.